JP7724370B2 - Drive system - Google Patents

Description

The following disclosure relates to a drive system used in a vehicle with multiple power sources to control the torque output to shafts corresponding to each power source.

In conventional vehicles, torque generated by a single power source is distributed to multiple drive wheels. A differential, for example, is used for this purpose.

Without a differential limit, when one shaft spins freely, it cannot output torque to the other shaft. Therefore, differentials are often equipped with a mechanism to limit or temporarily stop the differential. A mechanism that limits the differential, for example, frictionally, is often called a limited slip differential (LSD), and a mechanism in which the differential limiting force changes dynamically in response to the input torque is called a torque-sensitive LSD. A mechanism in which the limiting force is controlled by a mechanical mechanism is called a mechanical LSD, and one that uses an electronic control device is called an electronically controlled LSD.

Patent documents 1 and 2 disclose related technologies.

International Patent Application Publication WO2010/078937A1 International Patent Application Publication WO2020/129400A1

Unlike gasoline engines, electric vehicles have relatively small power sources, allowing one power source to be placed at each drive wheel. Because torque can be controlled individually for each drive wheel, torque control makes it easier to prevent understeer and oversteer, and also makes it possible to achieve behaviors that are difficult to achieve in conventional vehicles, such as torque vectoring. However, for drivers who are particularly skilled at driving conventional vehicles, the occurrence of unintended behavior does not necessarily make the vehicle easier to drive, and they may even find the behavior of a conventional powertrain that uses an LSD more preferable.

The system disclosed below is a vehicle drive system with multiple power sources, yet achieves behavior similar to a power system using an LSD.

According to one aspect, a drive system for controlling torque output to a first shaft and a second shaft, respectively, includes a first speed sensor configured to measure the rotational speed of the first shaft, a second speed sensor configured to measure the rotational speed of the second shaft, a first output device drivingly coupled to the first shaft, a second output device drivingly coupled to the second shaft, and an electronic control device electrically connected to the first and second speed sensors and the first and second output devices, the electronic control device including a memory device that stores one or more differential torque ratio values, and a target torque and an electronic control device configured to: obtain a torque value from the storage device, determine a rotational speed difference of the first shaft with respect to the second shaft, determine a sign from the determined rotational speed difference and the target torque value, read a value from the storage device, calculate a first allocation ratio to be allocated to the first shaft and a second allocation ratio to be allocated to the second shaft from the read value and the determined sign, and control the first output device and the second output device to output torques obtained by multiplying the calculated first and second allocation ratios by the target torque value, respectively.

FIG. 1 is a diagram showing a schematic diagram of an example of a vehicle to which a drive system according to this embodiment is applied, in which the right and left rear axles are driven by respective power sources. FIG. 2 is a schematic diagram showing an example in which the front axle is driven by a motor. FIG. 3 is a schematic diagram showing an example in which the drive system controls the front power source and the rear power source. FIG. 4 is a schematic diagram showing an example in which the drive system controls a front power source and a rear power source, and the front power source is an engine. FIG. 5 is a schematic diagram showing an example in which all axles are driven by their own power sources. FIG. 6 is a cross-sectional view of an example of a gear arrangement used when one motor drives both axles. FIG. 7 is a diagram showing the torques that should be generated on the inner and outer wheels when the vehicle turns a curve, and shows the driving state in which the vehicle is driven forward. FIG. 8 is a diagram showing the torques that should be generated on the inside and outside wheels when a vehicle turns a curve, and shows a coasting state in which the vehicle moves forward by inertia. FIG. 9 is a diagram that schematically shows the calculations performed by the electronic control unit. FIG. 10 is a graph showing an example of a function used for input conversion. FIG. 11 is a flowchart of the calculations and controls executed in the electronic control unit.

Several exemplary embodiments are described below with reference to the accompanying drawings.

For example, referring to FIG. 1, the drive system according to this embodiment can be applied to a vehicle 1 in which the right and left rear axles are each driven by a separate output device 7. The drive system controls the torque output to the right and left axles. In this case, the front axle may be a non-drive axle, or may be driven by a power source 3, such as an engine or a hybrid system equipped with both an engine and an electric motor. The power source 3 is combined with a gear system including a transmission 5 that transmits torque to the front axle. Alternatively, the drive system can be applied to a vehicle in which the front axle, instead of the rear axle, is driven by a separate output device 7. As a further alternative, as shown in FIG. 2, the front axle may be driven by an electric system 9 equipped with an electric motor.

Alternatively, instead of controlling the torque output to the right and left axles separately, the front and rear axles may each be equipped with a power source, and the drive system may be used to adjust the torque output to the front and rear axles by controlling the front and rear power sources. Figure 3 shows an example of such a vehicle 1, in which both the front and rear axles are driven by an electric system 9. Alternatively, this embodiment can also be applied to a vehicle in which one of the front and rear axles is driven by a power source 3 equipped with an engine or both an engine and an electric motor.

Furthermore, as illustrated in Figure 5, the drive system of this embodiment can be used to control the torque of the right front axle and left axle in addition to the right rear axle and left rear axle, i.e., all axles.

Referring to any of Figures 1 through 5, each ECU has a storage device that stores commands and data, and a microcontroller that can read these from the storage device and execute the commands. A typical vehicle has dozens of ECUs, but Figures 1 through 5 show only six of them. These ECUs 13 through 17 read the status of each part of the vehicle using various sensors and are connected to each other by a bus 19. ECUs 13 through 17 communicate and share information, for example, via a so-called controlled area network (CAN). This information includes not only the read status but also requests to other ECUs, and each ECU uses this information to control each part of the vehicle.

The ECU 13 is electrically connected to, for example, the console, steering wheel, accelerator pedal, brake pedal, etc., and receives input from the driver through these and controls them. Of course, instead of relying on a single ECU, each independent ECU may be responsible for information acquisition and control. The vehicle 1 also has multiple speed sensors 11 that measure the rotational speed of each axle, and the ECU 15, for example, is electrically connected to these and receives their output to calculate the rotational speed. For example, the ECU 17 is electrically connected to the power source 3, transmission 5, output device 7, and/or electric system 9, and controls their operation.

Referring to FIG. 6, the electric system 9 includes, for example, an electric motor 23, a reduction gear set 25 that reduces the rotation generated by the electric motor 23 before transmitting it, and a differential 27 that distributes torque to both axles. The output device 7 has a similar configuration and includes an electric motor and a reduction gear set, but does not include the differential 27, and the reduction gear set directly meshes with the gear portion of the output shaft. Here, the reduction gear set 25 is not required, but is advantageous for increasing the output of the electric motor 23. The entire electric system 9 or the entire output device 7 can be housed in a single casing 21, or one or more of these elements may be separate.

The electric motor 23 is, for example, a known inverter-driven motor. When power is input to the coil, it generates torque around the shaft according to the input, causing the rotor shaft 31 to rotate. The rotor shaft 31 can be a hollow shaft, and can be arranged coaxially with the differential 27, with one axle extending through its hollow 31H. Such a coaxial arrangement is advantageous for reducing the overall dimensions of the electric system 9. Of course, the electric motor 23 and both axles may be on different axes. Gear teeth are formed near the end of the rotor shaft 31, and rotation is output to the reduction gear set 25 via this gear portion 31G.

The reduction gear set 25 includes, for example, a gear shaft 35 having gear portions 33 and 35G. Gear portion 33 meshes with gear portion 31G of the rotor shaft 31, and gear portion 35G meshes with gear portion 37 of the differential 27. Gear portion 33 can have a larger diameter than gear portion 35G, so that the reduction gear set 25 can output a reduced rotation (i.e., increased power).

The differential 27 includes a casing 41, which includes a gear portion 37 on its outer circumferential surface and supports a differential gear set therein. The differential gear set includes a pair of side gears 43R, 43L, each of which has an engaging means such as a spline on its inner surface and is coupled to the right rear axle and left rear axle, respectively. In other words, the differential 27 distributes torque received via the gear portion 37 to the right rear axle and left rear axle via the side gears 43R, 43L.

Although not required in this embodiment, the casing 41 may be composed of an outer casing 41A and an inner casing 41B that is coaxial with the outer casing 41A and can rotate relative to it. When the actuator 29 is driven to engage the inner casing 41B with the outer casing 41A, the two rotate together and transmit torque to the differential gear set. When disengaged, torque is not transmitted.

The torque control aspects of the drive system according to this embodiment will be described with reference to Figures 7 to 10. In the following description, torque distribution to the left and right axles will be illustrated as an example, but needless to say, the following description also applies to torque distribution to the front and rear axles.

When the vehicle is moving forward (drive) with the power source exerting a driving force in the forward direction, if the driver turns the steering wheel, for example, to the left, the vehicle will follow a trajectory as shown by the arrow DT, as shown in Figure 7. At this time, the left rear wheel rotates at a slower speed than the right inner wheel. In a conventional powertrain using an LSD, the torque T _L distributed to the left rear wheel is greater than the torque T _R distributed to the right rear wheel. When considering simulating this, the torque distribution ratio can be calculated based on the virtual lock ratio f _LSD of a virtual LSD.

Referring primarily to Figure 9, if the ratio of torque T _L to torque T _R is defined as the differential torque ratio TBR, then the virtual locking ratio f _LSD required to achieve the target TBR is expressed by the formula (TBR-1)/(TBR+1). Therefore, the torque distribution ratio LH _Ratio to the left rear wheel is 1/2(1+f _LSD ), and the torque distribution ratio RH _Ratio to the right rear wheel is 1/2(1-f _LSD ). These values multiplied by the target torque value T _FrRg are the torques to be output to the left and right rear wheels, respectively, and needless to say, the sum of both is equal to the target torque value T _FrRg .

As can be easily understood, when the vehicle turns to the right, the torque distribution ratio is reversed, with the torque distribution ratio LH _Ratio to the left rear wheel being 1/2 (1- _fLSD ) and the torque distribution ratio RH _Ratio to the right rear wheel being 1/2 (1+ _fLSD ).

Whether the vehicle is turning right or left can be determined by the steering direction, or by the sign of the difference in rotational speed ΔN _RL between the right and left axles. If we refer to ΔN _RL , ΔN _RL > 0 when turning left, ΔN _RL = 0 when traveling straight, and ΔN _RL < 0 when turning right. If these are converted into signed binary values and reflected in the value of k (k = 1, 0, -1), the torque distribution ratio F _LSD to the left rear wheel can be expressed as 1/2(1 + k·f _LSD ) (see the right column in Figure 9). That is, LH _Ratio = 1/2(1 + k·f _LSD ) and RH _Ratio = 1/2(1 - k·f _LSD ).

On the other hand, as shown in Figure 8, when the vehicle is coasting, engine braking or regenerative braking is normally applied, causing the torques T _L and T _R to reverse direction (signs are reversed) and their magnitudes to also be reversed. Whether the vehicle is in the driving or coasting state can be determined from the sign of the target torque value T _FrRg . To reflect the sign reversal due to driving/coasting in the torque distribution ratio F _LSD , the product of the rotational speed difference ΔN _RL and the target torque value T _FrRg is converted to a signed binary value and can be reflected in the value of k (k = 1, 0, -1) (upper center of Figure 9).

Furthermore, different differential torque ratios TBR may be used depending on whether the vehicle is in drive or coast mode. Therefore, the storage device of the ECU 17 may store multiple different TBR _Drive and TBR _Coast values, and one of the values may be selected and used depending on whether the vehicle is in drive or coast mode. That is, referring to the lower left corner of FIG. 9 , when the target torque value _TFrRg is a positive value, the forward TBR _Drive may be used as the TBR, and when the target torque value TFrRg is a negative value, the reverse TBR _Coast may be used as the TBR. Different optimal values may be used for the forward TBR _Drive and the reverse TBR _Coast . An example of such values is a forward TBR _Drive of 2.7 and a reverse TBR _Coast of 2.3, but the present invention is not limited to these.

However, when ΔN _RL and _TFrRg are close to zero, even a slight disturbance will cause a change in sign. This can lead to a sudden change in the value of k, potentially causing unstable behavior of the drive system. Therefore, one or both of ΔN _RL and _TFrRg may be filtered using a low-pass filter or other method to remove noise. Alternatively, or in addition, the values may be transformed using an appropriate function to smooth out sudden changes in sign. Figure 10 shows an example of such a function, the so-called hyperbolic tangent function.

The ECU 17 can obtain the values necessary for the above determinations and calculations at any time through CAN communication via the bus 19. The ECU 17 controls the left and right output devices 7 to output torques obtained by multiplying the calculated distribution ratios LH _Ratio and RH _Ratio by the target torque value T _FrRg .

In the above explanation, the torque output to both drive shafts is controlled, and each motor is controlled so that the total torque equals the target torque value _TFrRg . When torque is distributed by a differential, if there is no slippage in the drive wheels, the average peripheral speed of both drive wheels should equal the vehicle speed. If only the output torque is controlled and the rotation speed of each motor deviates from this constraint, the vehicle may behave in an unexpected manner. Therefore, in addition to controlling the torque of each motor, the rotation speed of each motor may also be controlled.

Furthermore, rotational speed control may be used to limit the difference in rotational speed between the two drive wheels, or control may be used to make both drive wheels rotate at the same speed under certain conditions. These methods more closely simulate the behavior of an LSD and are advantageous for preventing wheel spin, for example.

The rotation speed may be controlled constantly, or only under certain conditions. In the former case, it is possible to control the rotation speed to stay within a range that satisfies vehicle speed constraints, for example by setting appropriate upper and lower limits. In the latter case, it is possible to set appropriate conditions and limit the rotation speed only when those conditions are met, or to switch to rotation speed control instead of torque control.

The vehicle speed may be calculated from the average value of the outputs of the speed sensors 11 on the left and right drive axles, or the average value of the outputs of the speed sensors 11 on all axles. Alternatively, other appropriate values, such as the integrated value of the acceleration sensor output, may be referenced. These values can also be obtained by the ECU 17 at any time via CAN communication via the bus 19.

The above calculations are standardized and can be performed by an independent calculation circuit without relying on the ECU 17. In other words, an external calculation circuit may execute the above calculations under the control of the ECU 17.

The control procedure by ECU 17 will be explained with reference to Figure 11. ECU 17 first obtains numerical values such as the target torque and the rotational speed of each axle using CAN communication, etc. From these numerical values, ECU 17 calculates the target torque, determines the direction the vehicle is turning, and decides whether to drive or coast. Next, ECU 17 determines the sign from numerical values such as the rotational speed difference and reflects this in the value of k. Next, ECU 17 determines TBR by selecting from a pre-stored data table or according to set conditions. ECU 17 uses these to calculate a virtual lock ratio, and calculates the torque distribution ratio to both shafts from the calculated virtual lock ratio. ECU 17 adjusts the power applied to the motor so that the torque obtained by multiplying the calculated torque distribution ratio by the target torque value is output to the output device of each shaft. These procedures are constantly repeated.

According to the above-described embodiment, even in a vehicle in which a power source is arranged for each drive wheel, the torque of each drive wheel can be controlled so that the vehicle behaves in a manner similar to that of a conventional vehicle. This makes it possible to provide a driving experience that is natural even for drivers who are accustomed to driving conventional vehicles. Furthermore, because the parameters to be adjusted correspond to the parameters of conventional vehicles, it is convenient to utilize knowledge gained in the design of conventional vehicles, meaning that this embodiment is also advantageous in terms of ease of vehicle design.

Several embodiments have been described, but modifications or variations of the embodiments may be made based on the above disclosure.

Claims (5)

A drive system that controls torque output to a first shaft and a second shaft, respectively, comprising:
a first speed sensor configured to measure a rotational speed of the first shaft;
a second speed sensor configured to measure the rotational speed of the second shaft;
a first output device drivingly coupled to the first shaft;
a second output device drivingly coupled to the second shaft;
an electronic control device electrically connected to the first and second speed sensors and the first and second output devices, and including a storage device that stores one or more differential torque ratio values, the electronic control device being configured to: acquire a target torque value; determine a rotational speed difference of the first shaft with respect to the second shaft; determine a sign from the determined rotational speed difference and the target torque value; read a value from the storage device; calculate a first distribution ratio to be distributed to the first shaft and a second distribution ratio to be distributed to the second shaft from the read value and the determined sign; and control the first output device and the second output device to output torques obtained by multiplying the calculated first and second distribution ratios by the target torque value, respectively;
A drive system with The drive system of claim 1, wherein the first output device comprises a first electric motor electrically connected to the electronic control device and a reduction gear set that reduces the rotation generated by the first electric motor and transmits it to the first shaft, and the second output device comprises a second electric motor electrically connected to the electronic control device and a reduction gear set that reduces the rotation generated by the second electric motor and transmits it to the second shaft. The drive system of claim 1, wherein the electronic control unit is configured to calculate the first distribution ratio based on the formula 1/2{1+k(TBR-1/TBR+1)} and the second distribution ratio based on the formula 1/2{1-k(TBR-1/TBR+1)}, where TBR is the read value and k is the determined sign. A drive system as described in claim 1, wherein the memory device stores two or more differential torque ratio values, and the electronic control device is configured to select and read one of the stored values depending on the sign of the target torque value. The drive system of claim 1, wherein the electronic control device is configured to adopt the sign of the result obtained by multiplying the determined rotational speed difference by the target torque value as the determined sign.